I am working on sending mails to 50 clients at one go( basically at lead genration). I was able to send 11 mails until I used static body through SetHtmlBody() of my Batch class having Database methods. But the problem arises when I have to choose the template dynamically as we have a form having radio buttons.
Now the same error is coming: Too many email invocations : 11.

Can you show the source code with your invocation? Messaging.sendEmailMessage takes a list. It's usually possible to assemble multiple emails, but send them all in one invocation.
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bigassforceJan 18 '13 at 13:09

You are receiving this message because you will have called it 11 times in the same context. Depending on your use case, there are a couple of ways to circumnavigate this. If you really do need to call the sendEmail method more than 10 times, you could use a Batch Apex class and set your batch size to 1 (to 10 depending how you implement it). This would allow you to call the method 10 times per execute iteration. See this blog entry for details:

In summary, Messaging.sendEmail accepts a List, which means each call to it can include a large number of emails to send - you need to bulkify your send email calls just like you would in an apex trigger.